1. Field of the Invention
This invention relates to a plasma processing apparatus and a processing method making use of the same. More particularly, it relates to a plasma processing apparatus by which negative ions can be generated in a large quantity and also the negative ions can be made incident on an article to be processed to etch or clean the article to remove unwanted matter therefrom, and a processing method making use of the same. This plasma processing apparatus is preferably used in processes for producing semiconductor devices such as LSIs, optical devices such as optical disks and waveguides, and magnetic devices such as magnetic disks.
2. Related Background Art
In general, it is said to be important to lower a plasma temperature in order to form negative ions in a plasma processing apparatus. The relationship between the plasma temperature and the negative ion formation can be known from the probability of attachment of electrons to particles with respect to electronic energy, which is described in, e.g., Basic Data of Plasma Physics (Sanborn C. Brown, AIP press, 1993). It is seen from this publication that the cross section in which the attachment of electrons to, e.g., chlorine-type gas molecules and dissociation thereof has a peak at about 1 eV. Meanwhile, plasma used in usual semiconductor production processes has an electron temperature of 2 to 5 eV. Accordingly, in order to efficiently form negative ions, it is considered necessary to lower the electron temperature to a suitable temperature.
For example, the following two can be given as typical examples of plasma processing apparatuses that utilize negative ions.
(1) Method making use of time afterglow of plasma:
An apparatus disclosed in Japanese Patent Laid-open Application No. 8-181125 can be given as one example of an apparatus utilizing this method. In FIG. 5A, reference numeral 501 denotes a microwave power source; 502, a magnetic-filled coil; 503, a waveguide; 510, an article to be processed; 512, a stand for supporting the article to be processed (hereinafter referred to as "article supporting stand"); 514, a plasma; 531, a vacuum vessel; and 532, a high-frequency power source. In this method, the plasma 514, which is formed by pulse-modulating microwaves generated from the microwave power source 501 at a cycle of 10 to 100 microseconds, is made ON/OFF, and, in the period of plasma-OFF, the plasma temperature is lowered to form negative ions. Also, a high-frequency bias is applied from the high-frequency power source 532 to the supporting stand 512 of the article 510 in synchronization with the pulse modulation of the plasma 514 to alternately draw out positive/negative ions into the article 510 to be processed as shown in FIG. 5B, thus the article 510 is processed.
(2) Method making use of space afterglow to spatially guide plasma downstream:
Not shown in the drawing, this method is a method which comprises placing the article to be processed downstream by tens of centimeters from the region of plasma formation and utilizing negative ions formed while being diffused downstream and cooled gradually.
The above two methods, however, have had the following problems.
i) In the method making use of pulse-modified plasma, positive ions are formed in the remaining half period where plasma is an ON state, and hence a high efficiency is promised for an etching apparatus of positive/negative-ion alternating irradiation. However, in the case of etching carried out by predominantly using negative ions, it is difficult to attain a high efficiency because the negative ions are formed only in the half of the processing time.
ii) In the method where plasma is guided spatially downstream to lower a plasma temperature to form negative ions, the recombination of plasma at vacuum vessel walls causes an abrupt decrease in plasma density itself, and hence the plasma formed in a high density can not efficiently be converted into negative ions.
Thus, in these conventional methods, there has been room for improvement in respect of large-quantity formation of negative ions and effective processing.